1. Field of the Invention
The present invention relates generally to corrosion and heat resistant alloys and, more particularly, is directed to a Fexe2x80x94Nixe2x80x94Cr alloy useful for diesel engine components, primarily exhaust valves. The alloy features a favorable balance of low cost, high-temperature monotonic and fatigue strength, corrosion resistance, and metallurgical stability. The alloy of the present invention may also be usefully employed in the manufacture of other diesel engine parts such as, for example, exhaust train components which experience similarly aggressive environments.
2. Description of the Prior Art
Heretofore, corrosion and heat resistant stainless steels such as 23-8N (Fe-23Cr-2.5Mn-8Ni-0.8Si-0.3C-0.3N) or 21-4N (Fe-21Cr-9Mn-4Ni-0.5C-0.4N) have been widely used for exhaust valves in low to medium performance diesel engines. For high performance engines, in contrast, expensive Ni-base superalloys such as NIMONIC(copyright) alloy 80A and alloy 751 have been implemented. Due to the ever-increasing demands on engine operating efficiency and reliability, in recent years the need for low cost, intermediate performance valve alloys has arisen.
With this goal in mind, recently several alloys such as Pyromet(copyright) 31V (Fe-56Ni-23Cr-2Mo-1.2Al-2.3Ti-0.8Nb-0.04C, U.S. Pat. No. 4,379,120), a 40 Ni alloy (Fe-41Ni-16Cr-0.9Al-2.8Ti-0.8Nb-0.05C, U.S. Pat. No. 5,567,383), and HI(copyright) 461 (Fe-47Ni-18Cr-1.2Al-4.0Ti-0.3C) were developed. Besides lowering the Ni content to the greatest extent possible without compromising the prerequisite strength requirements, special emphasis was placed on high-temperature abrasion resistance, thus eliminating the cost of expensive hardfacing.
Still, the aforementioned alloys exhibit some shortcomings. For example, Pyromet(copyright) 31V features a relatively high Ni content and was also found to precipitate a potentially embrittling acicular alpha (xcex1)-Cr phase after extended exposures to service temperatures of 760xc2x0 C. (1400xc2x0 F.). The 40 Ni alloy is low cost but contains only moderate amounts of Cr, thus impairing corrosion resistance. Furthermore, the alloy seems to be prone to unwanted eta (xcex7)-phase (Ni3Ti) precipitation upon extended exposure harming ductility. The most favorable balance between cost and performance has, seemingly, been attained with alloy HI(copyright) 461 which features a dispersion of primary TiC carbides in addition to the customary gamma prime (xcex3xe2x80x2)-strengthening. It was felt, however, that further performance enhancements at the same moderate cost level were still needed in order to achieve even further improvements in engine performance and reliability.
The present invention is directed to an improved alloy particularly suited for diesel engine exhaust valves and the like which features an attractive balance of low cost, high-temperature monotonic and fatigue strength, corrosion and abrasion resistance, metallurgical stability, and ease of fabrication.
The alloy according to the present invention is characterized by a composition in weight percent of about 0.15-0.65% C, 40-49% Ni, 18-22% Cr, 1.2-1.8% Al, 2.0-3.0% Ti, 0.9-7.8% Nb, not more than 1% Co and Mo each, the balance being Fe and inevitable impurities, whereby the Ti:Al ratio (wt. %) must not exceed 2:1, and the Nb:C ratio (wt. %) is adjusted to lie within the range of 6:1 and 12:1 (or 0.8:1 to 1.5:1 on an atomic basis). A further presently preferred Nb range is 0.9-6.5 wt. % with a Nb:C ratio of between 6:1 and 10:1 on a wt. % basis (or 0.8:1 to 1.3:1 on an atomic basis).
Furthermore, cost permitting, Nb may be partially substituted for Ta on an equiatomic basis. In this case, the ratio of the combined atomic percentage (Nb+Ta) to C present should be adjusted to lie within the range of 0.8-1.5.
The alloy may also contain certain elements essential for deoxidation/desulfurization and improved hot workability in the following amounts: up to 2.0% Mn, up to 0.01% B, and up to 0.3% Zr. Silicon additions up to 1.0 wt. % are also beneficial to improve the alloy""s oxidation resistance.
In a more presently preferred embodiment of the invention (in % by weight), the C content is limited to 0.25-0.55%, the Ni content is 42-48%, the Cr content is 19-21%, the Al content is 1.4-1.7%, the Ti content is 2.3-2.7%, the Nb content is 1.8-5.5%, the balance being Fe and inevitable impurities, and wherein the Nb:C weight % ratio is adjusted to lie within the range of 7:1 and 10:1, and the Ti:Al weight % ratio is less than or equal to 2:1. A still more preferred Nb range is about 2.5-3.0%.
The microstructure of the alloy of the present invention features, even after extended exposures to valve operating temperatures in the vicinity of 760xc2x0 C. (1400xc2x0 F.), essentially a uniform dispersion of micron size Nb-rich primary MC type carbides, fine discrete Cr-rich secondary M23C6 type carbides in the austenite grain boundaries, and submicroscopic intragranular xcex3xe2x80x2 precipitates. Moreover, the microstructure of a preferred embodiment of the invention features less than 5 vol. % of any acicular phase.
The present invention further includes diesel engine valves, particularly exhaust valves, as well as other exhaust train components, manufactured from the above described alloy.